RED SEA. 
261 
some of them are very long in proportion to their width. 
Captain Moresby informs me that any one who had not 
made actual plans of them; would be apt to think that they 
were much more elongated than they really are. Many of 
them rise to the surface, but the greater number lie from 5 
to 30 fathoms beneath it, with irregular soundings on them. 
They consist of sand and living coral ; the latter in most 
cases, according to Captain Moresby, covering the greater 
part of their surface. They extend parallel to the shore, 
and are not unfrequently connected in their middle parts 
by short transverse banks with the main land. The sea is 
generally profoundly deep quite close to them, as it is near 
most parts of the coast of the main land ; but this is not 
universally the case, for between lat. 15° and 17° the water 
deepens quite gradually from the banks, both on the eastern 
and western shores. In many parts islands rise from the 
banks ; they are low, flat-topped, and consist of the same 
horizontally stratified formation with that forming the 
plain-like margin of the main land. Some of the smaller 
and lower islands consist of mere sand. Captain Moresby 
informs me that small masses of rock, the remnants of 
islands, are left on many of the banks where there is now 
no dry land. Ehrenberg also asserts that most of the 
islets, even the lowest, have a flat abraded basis, com- 
posed of the same tertiary formation as elsewhere : he 
believes that as soon as the surf wears down the protube- 
rant parts of the banks to just beneath the level of the sea, 
the surface becomes protected from further abrasion by the 
growth of coral, and he thus accounts for the existence of so 
many banks standing on a level with the surface of this sea. 
It appears that most of the islands are certainly decreasing 
in size. 
The banks and islands are curiously shaped in the parts 
just referred to, namely, from lat. 15° to 17°, where the 
sea deepens quite gradually : the Dhalac group, on the 
